Heat exchangers used with recirculating exhaust gas are highly advantageous in that they reduce emissions in vehicles. Recycled exhaust must be cooled in order to achieve high efficiency during recirculation, especially to achieve better degrees of filling. However, it will be appreciated by those skilled in the art that the entire system (vehicle with internal combustion engine) and an overall energy balance are at issue. Toward that end, all operating phases in vehicles have heretofore been analyzed to account for alternating loads which may be encountered. One known measure to account for alternating loads involves bypassing the exhaust heat exchangers in phases in which cooling of the exhaust would be counterproductive (e.g., during the starting phases of the vehicle, which require an extremely large amount of fuel and in which the heat energy of the exhaust may be used to rapidly heat the engine to its optimal operating temperature). For example, bypassing the exhaust heat exchanger for such purposes is shown in European patent application/patents EP 916 837 (see also U.S. Pat. No. 6,213,105 B1) and EP 987 427, wherein an integrated valve in front of the exhaust entry to the exhaust heat exchanger allows the exhaust stream to be selectively diverted toward the exhaust heat exchanger or to bypassing the heat exchanger and passing directly into the recirculation line.
Additional solutions have been described in German Applications DE 197 33 964 A1 and DE 199 06 401 A1. In the first named document, a bypass line and the exhaust heat exchanger are separated from each other but both are arranged in a common housing. In the latter document, the bypass line passes around the exhaust heat exchanger on the outside without both being enclosed by a housing. In the exhaust heat exchangers themselves, so-called tube-bundle heat exchangers or coil-tube heat exchangers appear to be involved. Such heat exchangers are a special design which is not particularly compact or space-saving, and therefore disadvantageous in that respect if used in vehicle engine compartments where space is limited.
Exhaust heat exchangers have also been long used to heat the passenger compartments of vehicles, and have also generally required bypassing, among other things, because the heating demand is not permanently present. Such exhaust heat exchangers have also usually been of the tube-bundle type or coil-tube type, and include exhaust heat exchangers such as can be deduced from EP 942 156 A1 (see also U.S. Pat. No. 6,141,961), for example.
Additional solutions with integrated bypasses have been described in DE 101 42 539 A1 and in DE 199 62 863 A1, which disclose heat exchangers produced by demanding welding methods, are not particularly compact, and which require a fairly demanding bypass design.
The present invention is directed toward overcoming one or more of the problems set forth above.